Sample viscosity is often a critical parameter in the paint industry, food industry, pharmaceutical and cosmetic industries, plastics, polymer and petrochemical industries, medical/biochemical industries, etc., and devices for measuring the viscosity of fluids have been in existence for many years. One common viscometer type is the oscillatory viscometer, which measures sample viscosity by determining the amount of damping experienced by a moving probe immersed in a sample fluid.
J. G. Woodward, a pioneer in the field, developed one of the first oscillatory viscometers in 1953 (The Journal of the Acoustical Society of America, 25, 147-151, 1953, incorporated herein by reference). Woodward measured the amount of damping experienced by an oscillating plate immersed in a fluid, and developed a mathematical relationship between the damping and fluid viscosity. His calculations are still used today for oscillating probe viscometers.
Also in 1953, Roth and Rich described and later patented an apparatus and method for measuring the viscosity of fluid-like materials using an elongated strip of magnetorestrictive material vibrated by transducer coils. See J. Appl. Phys., Vol. 24, No. 7, 1953 and U.S. Pat. No. 2,839,915, both incorporated herein by reference. In both references a solid rectangular strip was used as a sample probe which oscillated in the longitudinal compressional mode.
In 1989, Portman (U.S. Pat. No. 4,799,378, incorporated herein by reference) disclosed a piezoelectric viscometer using, as a viscosity probe, a quartz reed with a ball formed on the end thereof. The vibratory motion of the piezoelectric element caused the probe to oscillate. The Portman viscometer, like the Roth viscometer, is a device wherein the viscosity probe's range of motion is approximately 1 mm or more, however, the measurement of viscosity in such a system depends upon viscous flow theory.
Several viscometers have been developed using probes which undergo a twist vibration (i.e., a torsional mode of vibration) about a central axis. Miura (U.S. Pat. No. 4,811,593, incorporated herein by reference), disclosed such an instrument in 1989 using a transmission shaft that, via twist vibration, detects the viscous resistance offered by a liquid sample. The Paar Physica U.S.A. Physica-Rheoswing.RTM. commercial rheometer uses such torsional oscillations in determining dynamic and kinematic viscosity.
Harada, also in 1989, described a rotating viscometer wherein a rotary member spins within a fixed housing, causing the sample whose viscosity is being measured to flow. See U.S. Pat. No. 4,811,593, incorporated herein by reference. The Bohlin Visco 88.RTM. viscometer utilizes this general configuration by providing a rotating inner cylinder and a stationary outer cylinder.
Even with this great diversity in viscometer design, however, no viscometer to date has successfully combined the attributes of simple design, portability, field toughness, low cost and accuracy. Portability and field toughness are particularly advantageous for applications which include the need to measure the viscosity of a sample outside of a laboratory setting, such as the viscosity of a drilling mud on an off-shore oil rig. Sending samples back and forth to a laboratory for viscosity determination can be tedious and time-consuming, and can significantly slow down work already in progress. That a viscometer needs to be accurate is self-evident. Low cost, of course, if it can be combined with portability and accuracy, is also clearly desirable. Unfortunately, current commercially available viscometers are expensive, ranging in cost from about $7,000 to $25,000.